Process and apparatus for injection molding of plastic materials

ABSTRACT

A method for injection molding of plastic material by means of at least one injector whose pin valve can be displaced between a fully closed position and a maximum controlled fashion according to a position and speed of the pin valve. During displacement of the pin valve from the fully closed position to the maximum open position, and/or vice versa, the pin valve is stopped in intermediate positions and the displacement speed thereof is uniform and constant. An initial opening step at maximum speed and one or more steps of inversion of the motion of the pin valve can be provided for.

FIELD OF THE INVENTION

The present invention refers to the injection molding of plasticmaterials and more in particular it regards a method for injectionmolding by means of an apparatus comprising a distributor of fluidplastic material under pressure connected to at least one injectorincluding a pin valve displaceable between a fully closed position and amaximum open position and vice versa.

Typically, these injection molding methods comprise a step for fillingthe mold cavity with the plastic material following the displacement ofthe pin valve from the fully closed position to the maximum openingposition, followed by a step of packing the plastic material injectedunder pressure into the cavity, whereby the pin valve is held in themaximum open position. The pin valve is then displaced from the maximumopen position to the fully closed position and the molded detail isremoved from the mold after a waiting period so as to allow the plasticmaterial to solidify.

The displacements of the pin valve or each injector are conventionallycarried out through a fluid actuator. Apparatus in which the pin valveis actuated by an electric actuator, for example rotary, have beenproposed in more recent times.

STATE OF THE PRIOR ART

Both electric and fluid actuators are typically controlled utilisingelectronic systems which operate according to process parameters,detected by means of special sensors, and/or by means of specificalgorithms. This allows to efficiently control both the position and thedisplacement speed of the pin valve between the closing position and theopening position so as to vary the flow of the plastic material injectedduring the molding cycle. Thus, documents WO-2012/074879A1 andWO-2012/087491A1 provide for controlling the actuator to displace thepin valve continuously from the closing position to the opening positionat one or more so-called intermediate speeds and then at one or morespeeds higher than the intermediate speed/s. Such control is carried outas a function of the time or of the space covered by the pin valvestarting from the closed position thereof: a position sensor directlyassociated to the pin valve which transmits the signals thereof to theelectronic control of the actuator is provided for in such case.

Document US-2015/266216A1 (U.S. Pat. No. 9,738,016B2) in the name of theApplicant provides for a discontinuous displacement of the pin valvefrom the closing position to the opening position, with at least onestep for stopping in a partial opening intermediate position. Thedisplacement speed of the pin valve is not an issue.

Even document US-2016/167272A1 in the name of the Applicant provides fora discontinuous displacement of the pin valve, which—during theopening—is driven at a first speed, then stopped in at least one partialopening intermediate position and subsequently driven at at least onesecond speed higher than the first speed. Even the displacement of thepin valve from the maximum opening position to the fully closingposition is discontinuous, i.e. it can provide for a step of temporarystop in a partial closing intermediate position, and the displacementspeed from this partial closing position to the fully closing positionis different, for example lower than the displacement speed of the pinvalve from the maximum opening position to the partial closingintermediate position.

This type of control is difficult to correlate with the actual processconditions, i.e. a series of considerably variable parameters forexample as a function of variation of the operating conditions and thephysical state of the plastic material, and also the pressure of theplastic material supplied to the injector. This can create surfacedefects on the molded pieces.

Should the actuator that actuates the pin valve be of the electricaltype and specifically of the electrical rotary type it is convenient,for simplicity purposes, to detect the position of the pin valve notdirectly but rather indirectly. In particular in the case of documentEP-2679374A1 in the name of the Applicant the shaft of the rotaryelectric motor actuates the pin valve of the injector by means of atransmission which distinctively includes a nut and screw unit and anoscillating lever. The position of the pin valve is in this caseconveniently detected by means of an encoder associated to the driveshaft and operatively connected to the electronic control thereof.Considering both the possible clearances present in the transmissionkinematic chain and the possible drive effects of the pin valve by theinjected material, the detection of the position of the pin valve duringthe opening and closing stroke indirectly carried out by means of theencoder would not be as precise as it would be desired for an accuratecontrol of the position and speed of the pin valve.

A more accurate and precise control is particularly necessary for theinjection molding of articles that require high quality both from amechanical point of view and from an aesthetic point of view.

SUMMARY OF THE INVENTION

The object of the invention is to provide an efficient solution to theaforementioned technical problem, through a peculiar control of theinjection process capable of allowing to be able to limit or entirelyeliminate the aforementioned defects from the molded pieces.

With the aim of attaining this object, the invention is directed to aninjection molding method of the type defined at the beginning, in whichduring the displacement from the fully closed position to the maximumopen position the pin valve is stopped in a plurality of partial openingintermediate positions and the displacement speed of the pin valvebetween the stop positions is uniform and constant.

According to another aspect, the method according to the inventionprovides for that the pin valve be displaced at a uniform and constantspeed even from the maximum open position towards the fully closedposition, temporarily stopping in a plurality of partial closingintermediate positions.

The opening and closing constant and uniform speed can be programmed asa function of the characteristics of the injected plastic material andthe articles to be molded, and the opening constant and uniform speedcan be equal to or different from the closing constant and uniformspeed.

The duration of the stop times of the pin valve can be constant or, morepreferably, variable. In particular, during the opening displacementthereof, the stop times shorten progressively as the pin valve drawscloser to the maximum opening position.

According to a further peculiar aspect of the invention, the openingdisplacement of the pin valve is not only discontinuous but it alsoprovides for one or more steps for the temporary and quick returntowards the closing position. The same mode can also be provided for inthe closing displacement of the pin valve which, besides beingdiscontinuous, may provide for one or more steps for the temporary andquick return towards the opening position. These steps can for examplebe provided for in proximity of an initial position and/or of anintermediate position and/or of a final position of the opening orclosing stroke of the pin valve essentially with the aim calibrating andpossibly compensating or correcting the signals regarding the indirectdetection of the position of the pin valve transmitted by the encoder ofthe electric motor to the electronic control unit. In this manner, thepin valve speed control becomes considerably more accurate, to theadvantage of the quality of the molded articles. It should be observedthat these steps for inverting the ascending (“upstream”) and/or thedescending (“downstream”) motion of the pin valve have always beenconsidered inappropriate, if not unwanted, by those skilled in the art.Hence, besides attaining the aforementioned advantages the oppositesolution thus proposed by the Applicant allows to clearly overcome aconsolidated technical drawback.

According to a further characteristic of the invention the initialopening speed of the pin valve starting from the fully closing positioncan be conveniently different and more in particular greater than theaforementioned uniform and constant displacement speed, and thus be amaximum opening speed, until the pin valve stops in a first minimumpartial opening position.

Thanks to the invention, the Applicant surprisingly found out that it ispossible to obtain considerable effects in terms of improved aestheticquality of the molded pieces particularly in case of molding apparatuswith multiple injectors actuated in a sequence or cascade fashion.

The invention also regards an apparatus for implementing the injectionmolding method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to theattached drawings, provided purely by way of non-limiting example,wherein:

FIG. 1 is a partially cross-sectional schematic view showing an injectorfor an injection moulding apparatus according to the invention,

FIG. 2 is a diagram showing an example of an opening cycle of the pinvalve according to the invention, in which the upper part of the figurerepresents the speed/time correlation and the lower part represents theposition/time correlation of the pin,

FIG. 3 is a diagram showing an example of a closing cycle of the pinvalve according to the invention, in which the upper part of the figurerepresents the speed/time correlation and the lower part represents theposition/time correlation of the pin valve,

FIGS. 4-8 are diagrams similar to that of the lower part of FIG. 2showing further examples of opening cycles of the pin valve of theinjector according to the invention, and

FIGS. 9-11 are diagrams similar to that of the lower part of FIG. 3showing further examples of closing cycles of the pin valve of theinjector according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The injection moulding apparatus subject of the invention is generallyconventional and well-known to the man skilled in the art. It can forexample be of the type described and illustrated in the previouslymentioned document US-2015/266216A1 (U.S. Pat. No. 9,738,016B2) andcomprise, in brief, a distributor of the molten plastic material or hotrunner which supplies the plastic material to one or more injectors eachcomprising a nozzle movable in which is a pin valve that can be axiallydisplaced, by means of an actuator, between an advanced fully closingposition and a receded maximum opening position so as to allow the flowof the plastic material into the mold, and vice versa to close the flow.In case of multiple injectors, the molding cycle may provide for, in aknown fashion, an injection of the sequential or cascade type.

The actuator that controls the displacements of the pin valve of the orof each injector can be of the fluid type (hydraulic or pneumatic) or ofthe electric type. An example of such electric actuator, of the rotarytype, is described and illustrated in the prior art documentEP-2679374A1 in the name of the Applicant, in which the electric driveshaft actuates the pin valve by means of a transmission including a nutand screw unit and an oscillating lever.

The actuator, whether of the electric or fluid type, is operativelyconnected to a programmable control electronic unit and configured toactuate the pin valve in a controlled fashion according to the positionand displacement speed thereof. The control can be programmed as afunction of operative parameters of the molding apparatus, such as forexample the characteristics of the plastic material and those of thearticles to be molded, the pressure of the plastic material injectedinto the cavity, and/or further pre-set parameters.

FIG. 1 schematically shows a molding apparatus thus made in which aninjector 3, connected to a distributer of molten plastic materialdistributor or hot runner 2, comprises a nozzle 5 provided—at the freeend thereof—with a nozzle terminal 6 placed in communication with thecavity of a mold by means of a gate. The flow of the plastic materialthrough the nozzle terminal 6 is controlled by a pin valve 7 axiallydisplaceable along the nozzle 5 by means of an actuator 8, between thelowered fully closing position represented in FIG. 1, and a raisedmaximum opening position.

The actuator 8 is in this case an electric actuator, and more inparticular a rotary electric motor: the arrangement thereof illustratedin FIG. 1 is solely for indicative purposes, in that it can convenientlybe of the type described and illustrated in the aforementioned documentno. EP-2679374, according to which the shaft of rotary electric motor isarranged transversely to the pin valve 7 and actuates it through atransmission (not illustrated) including a nut and screw unit and anoscillating lever. It should be observed that the invention alsoidentically applies to other configurations, for example of the type inwhich the shaft of the electric motor is axially aligned with the pinvalve 7.

The electric motor 8 is provided—in a known fashion—with an encoder 9associated to the shaft thereof and suitable to indirectly detect theposition of the pin valve 7. The encoder 9 is operatively connected toan electronic control unit 4 configured to actuate electric motor 8, andthus the pin valve 7, in a controlled fashion as a function of variousparameters. In particular, the displacement speed of the pin valve 7carried out by the electric motor 8 is carried out as a function of theposition thereof detected by means of the encoder 9.

According to the distinctive characteristic of the invention, theprogrammable control provides for the discontinuous displacement of thepin valve and the stopping thereof in one or more intermediate positionsbetween the fully closing and maximum opening, and/or vice versa, bysetting an always uniform and constant speed during each step fordisplacing the pin valve in the opening stroke thereof and/or in theclosing stroke thereof, unless as otherwise explained hereinafter. Suchuniform and constant speed combined with the displacement pauses of thepin valve surprisingly allows to improve the aesthetic and structuralqualities of the molded articles.

The diagram of FIG. 2 shows an example of an opening cycle according tothe invention: the upper part of the diagram represents the speed/timecorrelation and the lower part represents the position/time correlationof the pin valve 7. As observable, the displacement from the fullyclosing position to the maximum opening position provides for varioussteps for stopping the pin valve partial opening intermediate positions,in which the speed thereof is null. The displacement speed of the pinvalve between each stop position and the subsequent one, except foracceleration-deceleration transients, is always constant and uniform,and this speed can be programmed by means of the electronic control unit4.

The diagram of FIG. 3 shows an example of a closing cycle according tothe invention: also in this case the upper part of the diagramrepresents the speed/time correlation and the lower part represents theposition/time correlation of the pin valve. As observable, thedisplacement from the maximum opening position to the fully closingposition provides for various steps for stopping the pin valve inpartial closing intermediate positions, in which the speed thereof isnull. The speed of the pin valve between each stop position and thesubsequent one is always constant and uniform, and it can also beprogrammed, i.e. it can be modified whenever deemed necessary, by meansof the electronic control unit 4.

For example, the speed in the sections in which the pin valve is beingdisplaced can constantly be in the order of 20 mm/sec. This value, samecase applying to the number, the position and the duration of the stopsteps of the pin valve may vary depending on the programming mode of theelectronic control of the actuator of the injector, whether of theelectric or fluid type. Thus, the opening constant and uniform speed maybe equal to or different (greater or lesser) from the closing constantand uniform speed. Furthermore, the discontinuous displacement, withconstant and uniform displacement speed, of the pin valve may beprovided for only in one or in the other of the opening and closingcycles.

It should be observed that the constant and uniform displacement speedbetween a stop position and the subsequent one, neither considers thetransients required for accelerating the pin valve, starting from eachposition in which the speed thereof is null, until the standarddisplacement speed thereof is achieved, nor the transients required todecelerate the pin valve up to stopping it in each position in which thespeed thereof is null. These transients are to be considered negligible,and thus irrelevant to the effects of the invention and the definitionthereof outlined in the claims that follow, considering the extremelyshort times thereof both with respect to the entire pin valve openingand/or cycle time and with respect to the time interval between eachstop and the subsequent one.

FIGS. 4-8 and 9-11 show further alternative examples of speed profilesof the pin valve 7, respectively in opening and closing mode, which fallwithin the scope of protection of the present invention. In thesediagrams, the abscissas indicate the time in seconds or multiples whilethe ordinates indicate the position of the pin valve in mm. ormultiples.

In the profile of FIG. 4 the stop pauses between each displacement ofthe pin valve at uniform and constant speed have an identical duration,while in the case of FIG. 5 the pauses have a decreasing duration i.e.they shorten progressively towards the maximum opening position.

Provided for in the profile of FIG. 6 is a short initial section for thedisplacement of the pin valve at a speed greater than the uniform andconstant speed in the subsequent displacement sections before and aftereach pause stop. Thus, this initial speed is a maximum opening speed ofthe pin valve. Also in this case, the duration of the stop pauses of thepin valve decreases towards the maximum opening position.

According to the invention, during the opening cycle using the variousmodes described above there can also be provided for at least oneinversion of the displacement motion of the pin valve, i.e. at least onetemporary brief return thereof towards a partial re-closing position.This motion inversion is particularly advantageous should the detectionof the position of the pin valve be, like in the case described herein,carried out indirectly by means of the encoder 9 (or equivalentsystems), in that this allows to verify and possibly correct the signalsregarding the indirect detection of the position of the pin valve 7transmitted by the encoder 9 of the motor 8 to the electronic controlunit 4. In this manner, the precision of the control of the speed of thepin valve becomes considerably more accurate, to the advantage of thequality of the molded articles. Provided for in the case of FIG. 6 aretwo brief temporary inversion steps respectively in proximity of aninitial position and of a final position of the opening or closingstroke of the pin valve, while in the case of FIG. 7 there is providedfor a third one at an intermediate position of the opening stroke.

In the profile of FIG. 7 the duration of the stop pauses of the pinvalve is also variable and in the profile of FIG. 8 such durationextends towards the maximum opening position.

It should be observed that the profiles described and illustrated hereinare provided purely by way of non-limiting example: in particular, anyaggregation or combination thereof can be provided, still falling withinthe scope of protection of the present invention.

The same operating modes described above can be provided for in theclosing displacement of the pin valve which, besides beingdiscontinuous, has a uniform and constant displacement speed between thestop pauses, whose duration can be equal or different. Also, one or moresteps for the temporary and brief return to a partial opening positioncan be contemplated. Thus, the closing profile represented in FIG. 9provides for variable pauses and motion inversion respectively inproximity of the initial maximum open position and the final fullyclosed position. The profiles of FIGS. 10 and 11 on the contrary do notprovide for any inversion and the duration of the stops between eachdisplacement section at uniform and constant speed is respectively equalor different, in particular it increases towards the fully closingposition.

The stop times of the pin valve during the closing and/or openingdisplacement can be conveniently variable according to a mathematicalfunction referred to the internal conditions of the mold: for example,the duration of the stop can be proportional to the injection pressurein the various points of the mold and/or to the flow rate of theinjected material.

As previously clarified, the opening and closing profiles of the pinvalve can be configured independently from each other. Furthermore, theycan have, the one or the other, conventional profiles for example of theconstant and continuous speed type.

Obviously, the construction details and the embodiments of the apparatusand the relative injector described herein with reference to theexemplifying embodiment may widely vary without departing from the scopeof protection of the present invention as described in the claims thatfollow. Thus, though the invention has been described with reference toan electrical actuation of the injector, it can also be advantageouslyapplied to the case of fluid actuators, for example pneumatic, withelectronic control. In such case, the indirect detection of the positionof the pin valve may be obtained by means of a linear encoder. Directdetection of the position of the pin valve—obtained by means of a linearencoder, a Hall effect magnetic sensor, an optical sensor or equivalentsystems—can also be envisaged both in the case of fluid actuators and inthe case of electric motors.

1. A method for injection molding of plastic material into a mold cavityby means of at least one injector including a pin valve displaceablebetween a fully closed position and a maximum open position and viceversa, wherein the pin valve is operated by an actuator in a controlledfashion in connection with a position and speed of the pin valve betweensaid fully closed and maximum open positions, and wherein duringdisplacement of the pin valve from the fully closed position to themaximum open position, the displacement of the pin valve provides for atleast one step of inversion of motion thereof.
 2. The method accordingto claim 1, wherein the displacement of the pin valve provides for aplurality of steps of inversion of the motion thereof.
 3. The methodaccording to claim 2, wherein the displacement of the pin valve providesfor an initial step, an intermediate step and a final step of inversionof the motion thereof.
 4. The method according to claim 1, wherein adisplacement speed of the pin valve is uniform and constant.
 5. Themethod according to claim 1, wherein during displacement from themaximum open position to the fully closed position, the pin valve isstopped in a plurality of partial closing intermediate positions and adisplacement speed of the pin valve between said partial closingintermediate positions is uniform and constant.
 6. The method accordingto claim 4, wherein the uniform and constant displacement speed duringdisplacement from the fully closed position to the maximum open positionis equal to the uniform and constant displacement speed duringdisplacement from the maximum open position to the fully closedposition.
 7. The method according to claim 4, wherein the uniform andconstant displacement speed during displacement from the fully closedposition to the maximum open position is different from the uniform andconstant displacement speed during displacement from the maximum openposition to the fully closed position.
 8. The method according to claim4, wherein said uniform and constant displacement speed of the pin valvecan be modified.
 9. The method according to claim 4, wherein an initialopening speed of the pin valve starting from the fully closed positionup to a first stop in a partial opening position is different andgreater than said uniform and constant displacement speed.
 10. Themethod according to claim 5, wherein a time the pin valve is stopped inthe plurality of partial closing intermediate positions has a constantduration.
 11. The method according to claim 5, wherein a time the pinvalve is stopped in the plurality of partial closing intermediatepositions has a variable duration.
 12. The method according to claim 11,wherein a time the pin valve is stopped during closing and/or openingdisplacement is variable according to a mathematical function referringto internal conditions of the mold, including injection pressure and/orinjected material flow rate.
 13. An Apparatus for injection molding ofplastic materials, comprising: at least one injector having a pin valvedisplaceable between a fully closed position and a maximum openposition, wherein said pin valve is actuated by an electrical orpneumatic actuator and associated sensor for direct or indirectdetection of a position of the pin valve, wherein an electronic unitoperatively connected to said sensor controls operation of said electricmotor and/or pneumatic cylinder, and wherein said electronic unit isconfigured to carry out the method according to claim
 1. 14. Theapparatus according to claim 13, wherein said motor is a rotary orlinear electric motor and said sensor is a rotary or linear encoder, ora Hall effect magnetic sensor, or an optical sensor.